The present invention relates to a measuring device for the moisture content of granular or powder materials. It relates particularly to such measurement of materials in movement on a conveyor belt or in a pipe. The device is of the type comprising a capacitative probe furnished with at least two electrodes separated by an insulating zone and in contact with the material to be studied, the probe having a cylindrical body, a frame for supporting the capacitative probe, and a high frequency oscillator circuit associated with the electrodes in order to supply an alternating voltage of frequency variable with the moisture content.
It is already known to use capacitative probes for measurement of the moisture content of materials, particularly of materials in the form of granulates, such as materials serving for the formation of concrete objects or of soils. Thus, French Pat. No. 2,062,078 describes a solid capacitative sensor of prismatic shape installed permanently in the midst of the material of which it is desired to measure the moisture content. This known capacitative sensor is associated with a connecting cable of high frequency quality for supplying the sensor with alternating current of a frequency comprised between 10 and 100 MHz.
French certificate of addition No. 2,192,711 also describes a capacitative sensor of moisture content of the aforesaid type, in which the measuring cell is at least partly conical and is associated with the means adapted to convert the capacitative variation of the sensor into a variation in the frequency of the electrical alternating signal delivered by the sensor.
The capacitative sensors of known types are fairly well adapted to measurements in situ, for which the measuring probe is anchored to a fixed position in a floor, for example, or to measurements of moisture content of material without fines. These capacitative sensors have on the other hand drawbacks when it is a matter of measuring the moisture content of certain granular materials, particularly materials possessing fines, when these materials are in motion on a conveyor belt or in a pipe and pass in contact with the capacitative probe on the walls of which are placed the measuring electrodes. In fact, in this case, there often occurs a clogging of certain particles of the material which remain stuck to the body of the probe. Errors of measurement then ensue so that these probes operate poorly in practice.